Metallic-carbide-group VIII metal powder and preparation methods thereof

ABSTRACT

A transition metal carbide-Group VIII metal powder comprising discrete particles of a transition metal carbide and Group VIII metal wherein: substantially all of the particles have a size of at most 0.4 micrometer; the transition metal carbide is selected from carbides of the group consisting of tungsten, titanium, tantalum, molybdenum, zirconium, hafnium, vanadium, niobium, chromium, mixtures and solid solutions thereof; and the Group VIII metal is selected from the group consisting of iron, cobalt, nickel, mixtures and solid solutions thereof. Said powders are produced by heating an admixture comprising a finishing source of carbon (e.g., acetylene black), a source of a group VIII metal (e.g., Co 3  O 4 ), and a particulate precursor to a temperature of about 1173 K to about 1773 K for a time sufficient to form a transition metal carbide-Group VIII metal powder, wherein at least about 25% by weight of the carbide precursor is carburized in forming the transition metal carbide of the transition metal carbide-Group VIII metal powder. The particulate precursor generally contains less than 2.5% oxygen by weight and contains compounds which undergo carburization such as a transition metal (e.g., W), lower valence transition metal carbide (W 2  C) to form the transition metal carbide (e.g., WC)-Group VIII metal powder.

This application is a divisional application of U.S. patent applicationSer. No. 08/657,988, filed Jun. 4, 1996, now U.S. Pat. No. 5,746,803,issued May 15, 1998, the contents of which are hereby incorporated intothe present disclosure.

FIELD OF THE INVENTION

The invention relates to transition metal carbide-Group VIII metalpowders and methods for preparing said powders. The invention relates,in particular, to tungsten carbide-cobalt powders.

BACKGROUND OF THE INVENTION

Metallic carbide powders are used to make densified or sinteredproducts. For example, it is well-known that monotungsten carbide (WC)is useful in the manufacture of commercially worthwhile items such ascutting tools, tool dies, blast nozzles and drill bits. In producingsaid WC items, it is common for a tungsten carbide powder to be combinedwith a metal such as cobalt and, subsequently, densified into a WC/Cocemented carbide by heating when making said tools.

As the particle size of the metallic carbide-metal powder decreases, thedensified products generally exhibit improved properties such asincreased strength and improved wear resistance. However, due to theirhigh surface energy, if the particles are too small they may causeexaggerated grain growth to occur when forming a cemented carbide part.Exaggerated grain growth adversely affects properties such as strength.Grain growth can be controlled to some extent by addition of graingrowth inhibitors such as VC, Cr₃ C₂, or TaC or by starting with a WChaving a narrow size distribution.

Densified metallic carbide-metal products having improved properties(e.g., increased strength) are also generally achieved by homogeneouslyblending the metallic carbide and metal powders. Homogeneously blendingthe powders generally results in a more uniform microstructureresulting, in less defects such as large grains due to exaggerated graingrowth and pores in the densified body.

Monotungsten carbide is typically formed by the carburization of metaltungsten. Metal tungsten carburization processes typically make WCpowders having a particle size of about 0.8 micrometer and largerbecause of the difficulty in producing W metal much smaller than thissize. Tungsten metal typically cannot be made much smaller than thissize due to synthesis limitations and the tungsten powder beingpyrophoric.

Methods which have attempted to make more homogeneously blended andsmaller WC-cobalt powder (i.e., WC-cobalt mixed powder) include thefollowing. The article, "Production of WC Powder from WO₃ with Added Co₃O₄," by Ushijima, et al., published in the Japan Metal Society Journal,42, No. 9, pages 871-875 (1978), describes a method to produce WC-cobaltpowder by carbothermal reduction of WO₃ and Co₃ O₄ in the presence ofcarbon in the form of carbon black and hydrogen. The WC-cobalt mixedpowder formed by this method had a particle size of 0.6 micrometer orgreater.

Pollizotti et al., (U.S. Pat. No. 4,851,041) disclose a WC--Co powderproduced by reduction decomposition of a suitable mixed metalcoordination compound such as tris(ethylenediaminecobalt) tungstateresulting in an atomically mixed high surface area reactive intermediateproduct, followed by carburization reduction of the intermediate productin flowing CO/CO₂ gas. The WC--Co mixed powder is described as beingcomposed of multiphase composite particles which are larger aggregatescontaining WC grains (particles) having a size of 10 to 20 nm in amatrix of beta-Co/W/C solid solution.

S. Takatsu in Powder Metallurgy International, Vol. 10, No. 1, pages13-15, 1978, discloses a method to produce WC powder by reducing a mixedoxide of W and Co by reducing and carburizing with gaseous reagentsusing a rotary kiln. The mixed oxide is first reduced to metal in ahydrogen atmosphere, then carburized in a methane hydrogen gas mixture,and finally further treated in hydrogen or a methane-hydrogen gasmixture to remove excess carbon and convert W₃ Co₃ C to WC and Co. Ahomogeneous WC--Co mixed powder is disclosed having a mean particle sizeof greater than or equal to about 0.4 micrometer.

It is desirable to provide a metallic carbide-metal powder and processto manufacture said powder wherein the powder has a particle size lessthan about 0.4 micrometer.

SUMMARY OF THE INVENTION

A first aspect of the present invention is a method for preparing atransition metal carbide-Group VIII metal powder, the method comprises:

heating an admixture comprising:

a finishing source of carbon,

a Group VIII powder source of iron, cobalt, nickel or mixture thereofand

particulate precursor comprised of a metal containing tungsten and aprecursor carbide comprising a carbide of a transition metal selectedfrom the group consisting of: tungsten; titanium; tantalum; molybdenum;zirconium; hafnium; vanadium; niobium; chromium and mixture thereof,

to a temperature of from about 1173 K to about 1773 K under ahydrogen-containing atmosphere for a time sufficient to form thetransition metal carbide-Group VIII metal powder wherein at least 25% byweight of the precursor carbide is carburized and the transition metalcarbide-Group VIII metal powder contains an amount of Group VIII metalof at least about 0.25% to at most about 50% by weight of the transitionmetal carbide-Group VIII metal powder.

A second aspect of the invention is a method for preparing a transitionmetal carbide-Group VIII metal powder, the method comprises:

heating an admixture comprising:

a finishing source of carbon,

a Group VIII powder source of iron, cobalt, nickel or mixture thereofand

a particulate precursor comprised of a precursor carbide comprising acarbide of a transition metal selected from the group consisting of:titanium; tantalum; molybdenum; zirconium; hafnium; vanadium; niobium;chromium and mixture thereof,

a temperature of from about 1173 K to about 1773 K under ahydrogen-containing atmosphere for a time sufficient to form thetransition metal carbide-Group VIII metal powder wherein at least 25% byweight of the precursor carbide is carburized and the transition metalcarbide-Group VIII metal powder contains an amount of Group VIII metalof at least about 0.25% to at most about 50% by weight of the transitionmetal carbide-Group VIII metal powder.

A third aspect of the invention is a transition metal carbide-Group VIIImetal powder comprising a mixture having particles of a transition metalcarbide and Group VIII metal wherein:

at least 50% by number of the particles are discrete,

the particles have at most an average aspect ratio of about 1.5,

substantially all of the particles have a size of at most 0.4micrometer,

the transition metal carbide is a carbide selected from the groupconsisting of tungsten, titanium, tantalum, molybdenum, zirconium,hafnium, vanadium, niobium, chromium, and solid solution thereof,

the Group VIII metal is selected from the group consisting of iron,cobalt, nickel and solid solution thereof and

the transition metal carbide-Group VIII metal powder contains an amountof Group VIII metal

of at least about 0.25% to at most about 50% by weight of the transitionmetal carbide-Group VIII metal powder.

A transition metal carbide-metal powder produced by a method describedherein is useful to make coatings and sintered bodies displaying highhardness and good wear resistance properties. Suitable applications ofsaid coatings and bodies include, for example, drill bits, blastnozzles, dies, punches and knives.

DETAILED DESCRIPTION OF THE INVENTION

The first and second aspects of the invention are methods for preparinga transition metal carbide-Group VIII metal powder. The method comprisesheating an admixture comprised of a finishing source of carbon, a sourceof Group VIII metal and a particulate precursor to a temperature fromabout 1173 K to about 1773 K under a hydrogen containing atmosphere fora time sufficient to form a transition metal carbide Group VIII metalpowder, wherein at least about 25% by weight of the precursor carbide iscarburized in forming the transition metal carbide of the transitionmetal carbide-Group VIII metal powder. Herein, a Group VIII metal isiron, cobalt, nickel or mixture thereof. The carburization of theprecursor carbide is believed to play a role in the formation of powderproduct having a small particle size.

Suitable Group VIII metal sources include metals, solid solution metals,oxygen containing compounds (e.g., an oxide), nitrides and carbides ofNi, Co and Fe. Other suitable Group VIII metal sources include solidsolution metals and carbide alloys of the aforementioned Group VIIImetals and a transition metal selected from the group consisting of:tungsten; titanium; tantalum; molybdenum; zirconium; hafnium; vanadium;niobium; chromium and mixture thereof. Preferably the source of theGroup VIII metal powder is an oxide. The average particle size of thepowder is preferably less than about 20 micrometers, more preferablyless than about 10 micrometers, and most preferably less than about 5micrometers to preferably greater than about 0.5 micrometer.

The admixture desirably contains an amount of Group VIII metal sourcesufficient to make a transition metal carbide-Group VIII metal powderhaving a concentration of Group VIII metal of at least about 0.25% byweight of said powder. Preferably the amount of group VIII metal sourceis sufficient to produce a transition metal carbide-Group VIII metalpowder having a Group VIII metal concentration of at least about 0.5%,more preferably at least about 1%, and most preferably at least about 2%to preferably at most about 50%, more preferably at most about 30%, evenmore preferably at most about 20% and most preferably at most about 15%by weight of the transition metal carbide-Group VIII metal powderproduced.

The finishing source of carbon, in the admixture, is a separately addedcarbon, residual carbon from the formation of the particulate precursoror mixture thereof. The separately added carbon suitably includes thosedescribed hereinafter for a reducing source of carbon. Preferably theseparately added carbon is a solid particulate carbon. More preferablythe separately added carbon is a carbon black and most preferablyacetylene black.

The finishing source of carbon is preferably present in an amount whichresults in a transition metal carbide-Group VIII metal powder havingminimal or no free carbon after heating (reacting) the admixture. Theamount of carbon advantageously ranges from 60% to 120% of thestoichiometric amount. The stoichiometric amount of carbon is the amountof carbon which would react with the oxygen to form carbon monoxide(i.e., reduction reaction) in the particulate precursor (e.g., WO_(x)),and Group VIII metal source (e.g., CO₃ O₄) and also carburize thetransition metal compounds (e.g., W, W₂ C, WO_(x)) in the particulateprecursor to a carbide of desired stoichiometry (e.g., WC) in theabsence of another reducing agent such as hydrogen, wherein "x"represents the amount of oxygen in the particulate precursor asdetermined by combustion analysis. When carbon is used in excess of thestoichiometric amount, a product containing little or no free carbon canstill be formed due to the loss of carbon from reaction with hydrogen(e.g., formation of methane).

The Particulate Precursor

Particulate precursor of the first aspect:

In the first aspect, the particulate precursor is comprised of a metalcontaining tungsten and a precursor carbide. The metal containingtungsten is suitably tungsten or a solid solution of tungsten and atransition metal selected from the group consisting of titanium;tantalum; molybdenum; zirconium; hafnium; vanadium; niobium; chromiumand mixture thereof. The tungsten containing metal is suitably presentin the particulate precursor in an amount of at least about 5% by weightof said precursor. The amount is preferably at least about 10%, morepreferably at least about 20%, and most preferably at least about 30% topreferably less than about 90% by weight of the particulate precursor.

The precursor carbide is suitably a transition metal carbide such as acarbide of Ti, W, Ta, V, Hf, Nb, Zr, Mo and Cr, wherein the valence ofthe transition metal is 2, 3 or 4 and the valence of the carbon is -4.For example, the carbide is preferably WC, W₂ C, or mixture thereof,when forming a monotungsten carbide-Group VIII metal powder andspecifically when forming a monotungsten carbide-cobalt powder. Theprecursor carbide is also suitably a solid solution transition metalcarbide such as (W,Ti,Ta)_(x) C; (Ti,Ta)_(x) C; (W,Ti)_(x) C or(W,Ta)_(x) C wherein "x" is 1 to 2. Desirably at least about 25% byweight of the precursor carbide is comprised of a transition metalcarbide, wherein the valence of the transition metal in said carbide islower than the valence of the transition metal in the followingcarbides: monotungsten carbide (WC), monotitanium carbide (TiC),monotantalum carbide (TaC), monovanadium carbide (VC), monohafniumcarbide (HfC), mononiobium carbide (NbC), monozirconium carbide (ZrC),dimolybdenum carbide (Mo₂ C), trichromium dicarbide (Cr₃ C₂) or solidsolutions thereof. More preferably the amount of lower valence carbideis at least about 30%, and even more preferably at least 40%, and mostpreferably at least about 50% by weight of precursor carbide.

The precursor carbide is desirably present in the particulate precursorin an amount of at least about 20% by weight of the particulateprecursor. Preferably the amount is at least 30%, more preferably atleast 35%, and most preferably at least about 50% to preferably at mostabout 90% by weight of the particulate precursor.

The particulate precursor may also contain a Group VIII metal which istypically in a reduced form. For example, the Group VIII metal can be inthe form of a metal, a metal in a metal solid solution, a carbide or acarbide alloy such as Co₆ W₆ C and Co₂ W₄ C when forming, for example, aWC--Co powder. The particulate precursor can also contain free carbon.The free carbon generally is a residue from the formation of theparticulate precursor described hereinafter.

In a preferred embodiment of the first aspect of the invention in whichWC-cobalt metal powder is formed, the particulate precursor desirablyconsists of tungsten, ditungsten carbide and monotungsten carbide. Thetungsten is typically present in an amount of from about 25 to about 70weight percent, more typically from about 40 to about 60 weight percent;ditungsten carbide is typically present in an amount of from about 25 toabout 70 weight percent, more typically from about 40 to about 60 weightpercent and monotungsten carbide is typically present in an amount offrom about 5 to about 50 weight percent, more typically from about 15 toabout 40 weight percent, based on the weight of the particulateprecursor.

To minimize or avoid formation of water vapor which may cause unwantedparticle growth during heating (reacting) of the admixture, theparticulate precursor preferably has an oxygen content of less thanabout 2.5, more preferably less than about 2, and most preferably lessthan about 1% by weight of the particulate precursor. To facilitate theproduction of a transition metal carbide-Group VIII metal powder havinga small size, the particulate precursor desirably has a particle sizethat is less than or equal to about 1.0 micrometer in diameter. Saidparticles are preferably at most about 0.5, more preferably at mostabout 0.4 micrometer, and most preferably at most about 0.2 micrometerto preferably at least about 0.01, more preferably at least about 0.02,and most preferably at least about 0.05 micrometer in diameter.

Particulate precursor of the second aspect:

The particulate precursor of the second aspect of the invention iscomprised of a precursor carbide of a transition metal selected from thegroup consisting of: titanium; tantalum; molybdenum; zirconium; hafnium;vanadium; niobium; chromium and mixture thereof, wherein thestoichiometery of the particulate precursor is the same as described forthe first aspect.

The precursor carbide of the second aspect is the same as the precursorcarbide of the first aspect, except that the precursor carbide of thesecond aspect does not contain tungsten. That is to say, said precursorcarbide does not contain a metal or carbide containing tungsten.

The precursor carbide can comprise all of the particulate precursor but:preferably is present in an amount less than 100% to an amount greaterthan about 50% by weight of the particulate precursor. For example, itis preferred that a transition metal(s) selected from the groupconsisting of: titanium; tantalum; molybdenum; zirconium; hafnium;vanadium; niobium; chromium and mixture thereof is present in an amountfrom about 1 to about 50% by weight of the particulate precursor. Saidtransition metal(s) can also be a solid solution metal of theaforementioned metals. The particulate precursor of this aspect of theinvention may also contain a group VIII metal and free carbon asdescribed for the particulate precursor of the first aspect. Inaddition, the particulate precursor of this aspect of the inventionpreferably has an oxygen and particle size as described hereinabove forthe first aspect of the particulate precursor.

Forming the Particulate Precursor

The particulate precursor can be formed by any convenient method such asreduction by carbon and/or hydrogen and carburization of an oxygencontaining transition metal compound, wherein the transition metal ofthe compound is tungsten, titanium, tantalum, molybdenum, zirconium,hafnium, vanadium, niobium, chromium or a mixture thereof. Herein,reduction is the removal of oxygen from a compound and carburization isdescribed hereinafter. Preferably said transition metal compound is atransition metal oxide, acid (e.g., tungstic acid) or ammonium compound(e.g., ammonium paratungstate). For preparing solid solution metalliccarbides, the transition metal oxide may be the oxide or oxides of morethan one of the transition metals listed above. The source of the oxidesor oxides of at least two transition metals may include separate oxidepowders of the two transition metals or a single multimetallic alloyoxide containing two or more of the transition metals. The transitionmetal oxide is preferably the simple oxide of the metal, such astungsten trioxide (WO₃), titanium dioxide (TiO₂) and tantalum pentoxide(Ta₂ O₅).

A desirable source of tungsten oxide has particles which are less thanor equal to 25 micrometers in diameter. A preferred particulate, WO₃, ofthis size is sold by GTE Products Corporation under the trade name"TO-3". Materials such as metatungstic acid, ammonium paratungstate orother tungsten oxides can be used in place of WO₃. "TITANOX™" fromVelsicol Chemical Corporation, Chicago, Ill., is a preferred source ofTiO₂. "TITANOX" is a trademark for Velsicol's series of white pigmentscomprising TiO₂ in both anatase and rutile crystalline forms. Some"TITANOX" series pigments are extended with calcium sulfate, but theseextended pigments are not preferred for use in the present invention. Apreferred source of Ta₂ O₅ is of less than 325 mesh (45 micrometers)size and greater than 99% purity, sold by Aldrich Chemical Company,Milwaukee, Wis. The oxides of the other metals should be of comparablepurity and particle size.

Preferably the particulate precursor is formed by heating theaforementioned transition metal compound with a reducing source ofcarbon to a temperature for a time under an atmosphere that isnon-oxidizing and free of hydrogen sufficient to reduce the transitionmetal compound into the particulate precursor. The temperature is atemperature where the formation of the desired transition metal carbideis thermodynamically favored.

The reducing source of carbon is preferably a particulate carbonmaterial such as carbon black or acetylene black. A particularlypreferred acetylene carbon black is commercially available from ChevronChemical under the trade designation "SHAWINIGAN". However, it iscontemplated that other carbon solid sources would also be suitable. Inaddition, other sources of carbon such as organic polymers,carbohydrates and hydrocarbons may be useful in place of all or part ofa particulate carbon material. Carbon black having a specific surfacearea of about 30 to about 90 m.sup. 2/g has been found to be suitablefor the invention.

The reducing source of carbon is used in an amount sufficient to formthe particulate precursor described hereinabove. The amount of carbon isdesirably an amount ranging from 60% to 120% by weight of thestoichiometric amount, the stoichiometeric amount being similar to thestoichiometeric amount for the particulate precursor previouslydescribed. That is to say, the stoichiometric amount of carbon is theamount of carbon which would react with the oxygen to form carbonmonoxide (i.e., reduction reaction) in the oxygen containing compound(e.g., WO₃), and Group VIII metal source if present (e.g., Co₃ O₄) andalso carburize the transition metal compounds (e.g., WO₃) to a carbideof desired stoichiometry (e.g., WC) in the absence of another reducingagent such as hydrogen.

A source of a Group VIII metal (i.e., Fe, Co, and Ni) can also beadmixed, heated and reduced along with the oxygen containing transitionmetal compound. A suitable Group VIII metal source and amount of saidsource is the same as those previously described. Preferably the sourceis an oxide of the Group VIII metal (e.g., NiO or Co₃ O₄).

The temperature is desirably equal to a temperature where the formationof the transition metal carbide having desired stoichiometry isthermodynamically favorable (i.e., the free energy of the reaction toform said carbide is negative). The reaction temperature must also beless than the melting point of any intended reaction product(s). Formono tungsten carbide, a reaction temperature of at least 1273 K isconsidered beneficial, while temperatures of from 1673 K to 2673 K arepreferred, and temperatures of from 1823 K to 2673 K are Tore preferred.When a heating rate of 10,000 K to 100,000,000 K per second is employed,via the entrainment method discussed herein below, a reactiontemperature of 1873 to 2423 K is satisfactory. Approximate minimumtemperatures at which the free energy of formation of the followingspecific reaction products is less than the free energy of formation ofcomponents of the finishing mixture needed to form the reaction productsare as follows: tungsten carbide (WC) 950 K; titanium carbide (TiC) 1555K; tantalum carbide (TaC) 1381 K; vanadium carbide (VC) 932 K; hafniumcarbide (HfC) 1934 K; niobium carbide (NbC) 1228 K; zirconium carbide(ZrC) 1930 K; dimolybdenum carbide (Mo₂ C) 742 K and trichromiumdicarbide (Cr₃ C₂) 1383 K.

The time at the reaction temperature during the reduction depends inpart upon the heating rate and reaction temperature, but must be highenough to reduce at least a major portion (i.e., desirably greater thanabout 90% by weight) of the transition metal compound containing oxygen.The time is preferably in the range of about 0.1 second to 1/2 hour,depending upon the heating method, heating rate, reaction temperatureand the ultimate particle size desired. Whatever combination of reactiontemperature, reaction time and heating rate is selected, however, itshould be adequate to convert said transition metal compound containingoxygen into the particulate precursor described previously.

The particulate precursor is preferably prepared by the rapidcarbothermal reduction method described below and described in moredetail in U.S. Pat. No. 5,380,688, incorporated herein by reference.

In preparing the particulate precursor by the method described by the'688 patent, an amount of reducing carbon (e.g., acetylene black) ismixed with a transition metal compound containing oxygen (e.g., WO₃).The amount of carbon used is the same as described before. An amount ofa group VIII metal (e.g., Co₃ O₄) source can also be mixed with saidcarbon and transition metal compound. The reactants (e.g., WO₃, C and,optionally, Co₃ O₄) can be mixed by any convenient technique such asV-blenders, jet mills and ball mills, the latter containing a suitablemilling media such as tungsten carbide-cobalt milling media.

The reactants are then heated advantageously at a rate of 100 to100,000,000 K/sec in a non-oxidizing atmosphere (i.e., rapidcarbothermal reduction). Generally, the heating rate for heating thereactants from room temperature to the reaction temperature ispreferably at least on the order of 100 to 10,000 K per second andoptimally on the order of 10,000 to 100,000,000 K per second.

The rapid carbothermal reduction can be performed by a drop orentrainment method as described in the '688 patent. In the drop method,the hot zone of an induction furnace is brought to the desired reactiontemperature, as described further hereinbelow, and allowed toequilibrate for 30 minutes under a flowing non-oxidizing gaseousatmosphere such as argon. Aliquots of the reactants (e.g., WO₃, C and,optionally, Co₃ O₄) are dropped into a graphite crucible in the hot zoneof the furnace. The extent of reaction is monitored by measuring thereaction by-product carbon monoxide level in the crucible as a functionof time. When the carbon monoxide level decreases back to its baselinevalue, it is assumed that the reaction is over. After the reaction isassumed to be over, the crucible and reactant products are cooled asrapidly as possible back to a temperature, such as room temperature,sufficient to minimize particle agglomeration and grain growth.

It has been determined that the rates of heating in this drop method arefrom about 100 K per second to about 10,000 K per second. In the dropmethod, typical preferred residence times are from about 5 minutes to 2hours for a reaction temperature of 1773 K with a heating rate of about100 to 10,000 K per second.

The rapid carbothermal process can be carried out by the entrainmentmethod as described in U.S. Pat. No. 5,380,688. The entrainment methodinvolves the use of a vertical graphite tube reaction furnace which isdisclosed in U.S. Pat. No. 5,110,565, incorporated herein by reference.The reactants are placed into a feed hopper, which allows flowingnon-oxidizing gas, such as argon, to entrain the mixture and deliver itto the furnace's reaction chamber as a dust cloud. The powder orparticulate mixture is immediately heated in the reaction chamber atrates of between about 10,000 to 100,000,000 K per second, while theaverage residence time of the particulate in the furnace is on the orderof seconds. In the entrainment method, a residence time of from about0.2 to 10 seconds for a reaction temperature of 1823 K or above with aheating rate of about 10,000 to 100,000,000 K per second is preferred.At the higher heating rate, residence times substantially greater than10 seconds may undesirably produce sintered aggregates rather thanparticulate product. As for exiting the hot zone of the reactionchamber, the flowing gas carries the powder into a water-cooledstainless steel jacket which rapidly cools reacted powder below 283 K.The entrainment method is the preferred method, as it has been shown toproduce smaller size particulates than the drop method.

It is believed that reaction temperature, residence time and heatingrate of the above methods are the main parameters controlling the sizeof the particles of the particulate precursor obtained. They do so byaffecting both the nucleation rate for forming the metal and metalcarbide particles and the growth rate of these particles once formed.For example, presuming that the particles are roughly spherical and theconversion of starting material to product occurs at a relativelyconstant volume rate, the growth rate of the particles should beproportional to the cube root of the residence time. In order tominimize the particle size of the resulting particulate precursor, thereaction temperature, heating rate and residence time should be selectedto yield a particle nucleation rate which is higher than, and preferablysignificantly higher than, the particle growth rate.

Forming the Transition Metal Carbide-Group VIII Metal Powder

To form a transition metal carbide-Group VIII metal powder, an admixtureof the particulate precursor, Group VIII metal powder source andfinishing source of carbon is heated to a temperature of from about 1173K to about 1773 K under a hydrogen-containing atmosphere for a timesufficient to form a transition metal carbide-Group VIII metal powder,wherein at least 25% by weight of the particulate precursor iscarburized to form the transition metal carbide-Group VIII metal powder.Said heating and subsequent carburization is referred to, hereinafter,as finishing or finishing reaction. The admixture of particulateprecursor, Group VIII metal source and finishing carbon is referred to,hereinafter, as the finishing mixture.

During finishing, carburization of the precursor carbide occurs.Carburization, herein, is the chemical bonding of carbon to anotherelement such as a carbon species reacting with a transition metalforming a carbide (e.g., W+C=WC or W₂ C ) and carbon species reactingwith a transition metal of a carbide, subsequently, forming a carbidewherein the transition metal has a higher valence (e.g., W₂ C+C=WC).During finishing, reduction by hydrogen and/or carbon (e.g., WO₃ +3H₂=W+3H₂ O; WO₃ +3C=W+3CO) may also occur. Elimination of carbon may alsooccur by the reaction with hydrogen to form, for example, methane.Preferably at least two of the above-described reactions occur duringfinishing. More preferably all of the above reactions occur duringfinishing.

Mixing to form the finishing mixture may be done by any convenientmixing techniques such as those described previously, ribbon blenders,roller mills, vertical screw mixers and fluidized zone mixers such asthose sold under the trade designation "FORBERG".

The finishing mixture can be static or moving during the finishingreaction. Preferably the finishing reaction is carried out by tumblingthe finishing mixture in a rotary graphite crucible reactor. Otherapparatus suitable for imparting motion to the finishing mixture duringheating include a rotary calciner, fluidized bed and vibrating fluidizedbed. The heating of the finishing mixture can be carried out in a numberof ways, for example, by induction heating of the rotary graphitecrucible.

The hydrogen-containing atmosphere advantageously contains at leastabout 1 mole percent hydrogen with the balance being an inert gas suchas argon. An atmosphere containing from about 3 to about 7 mole percenthydrogen in argon is particularly suitable. It is preferred that theatmosphere be a flowing atmosphere in order to carry away the gaseousby-products such as carbon monoxide and water vapor.

The temperature Of reaction during the finishing reaction is typicallyfrom about 900° C. (1173 K) to about 1450° C. (1723 K). The temperatureof the reaction may be used to manipulate the particle size of theproduct wherein a higher temperature generally leads to a product havinga larger particle size. The finishing step is, typically conducted for aperiod of time from about 10 minutes to about 2 hours. The lower thetemperature that is used to carry out the finishing reaction, the longerthe time will be to form the transition metal carbide-Group VIII metalpowder.

The Transition Metal Carbide-Group VIII Metal Powder Formed

The finishing step is conducted until the finishing mixture forms aproduct which is at least 95% by weight a transition metal carbide-GroupVIII metal powder. More preferably the product is at least about 98% byweight a transition metal carbide-Group VIII metal powder. Mostpreferably the product is at least about 99 percent by weight transitionmetal carbide-Group VIII metal powder. Impurities may be present in thetransition metal carbide-Group VIII metal powder such as elementaltransition metal, free carbon or transition metal-Group VIII-carbonalloys such as Co₆ W₆ C and Co₂ W₄ C. The powder desirably contains verylittle free carbon such as less than about 0.2% of the total powderweight. Preferably the free carbon is at most about 0.15%, morepreferably at most about 0.1%, and most preferably at most 0.05% byweight of the total powder. Preferably, the amount of transition metaland alloy impurity is below the powder X-ray diffraction detection limitas described in Elements of X-ray Diffraction, B. D. Cullity,Addison-Wesley, Reading Mass., 1956, relevant portions incorporatedherein by reference.

The transition metal carbide-Group VIII metal formed by theaforementioned methods are comprised of substantially discrete particlesof a transition metal carbide and discrete particles of a Group VIIImetal, wherein substantially means at least about 50% of the particlesby number are discrete particles. A particle is discrete when it isunconnected to any other particle. Preferably the number of particlesthat are discrete is greater than about 60, more preferably greater thanabout 75, even more preferably greater than about 90, and mostpreferably greater than about 95% by number. Said particles areuniformly and intimately mixed in the transition metal carbide-GroupVIII metal powder. The amount of particles that are discrete can bedetermined directly by electron microscopy.

Generally, the particles of the transition metal-Group VIII metal powderare equiaxed. Herein, equiaxed describes particles having an averageaspect ratio of at most about 1.5 wherein the aspect ratio is the ratiobetween the longest and shortest dimension of a particle as measured byelectron microscopy. Preferably the average aspect ratio is at mostabout 1.2. Said particles desirably have a particle size in whichsubstantially all of the particles are at most about 0.4 micrometer indiameter. Preferably substantially all of the particles are at most 0.3and more preferably are at most 0.2 micrometer in diameter. Saidparticles also preferably have a particle size in which substantiallyall of the particles are at least about 0.01, more preferably at leastabout 0.05, and most preferably at least about 0.1 micrometer indiameter. The aforementioned aspect ratio and particle size can bedetermined by direct measurement of a number of particles using electronmicroscopy. Substantially all, as just used herein, equates to at leastabout 95% by number of the particles falling within the specified sizes.It is also preferred that essentially all of the particles fall withinthe just specified particle sizes. Essentially all, as just used herein,equates to at least about 99% by number of the particles falling withinthe specified particle sizes.

The transition metal carbide of the powder is selected from carbides ofthe group consisting of tungsten, titanium, tantalum, molybdenum,zirconium, hafnium, vanadium, niobium, chromium, solid solutions thereofand mixtures thereof. Preferably the transition metal carbide ismonotungsten carbide (WC), WC containing solid solution such asWC--TiC--TaC or mixture thereof. Most preferably the transition metalcarbide is monotungsten carbide.

In a preferred embodiment, the powder is WC-cobalt powder having aparticle size of at most about 0.4 micrometer in diameter and a cobaltconcentration of at least about 1% by weight of the powder. Said powderis also preferred to have a particle size of greater than about 0.1micrometer in diameter. Said powder is even more preferred to have aparticle size of at most about 0.2 micrometer in diameter.

The following examples are illustrative only and should not be construedas limiting the invention in any way.

EXAMPLES

In the following examples, reference to "trace concentrations" equatesto less than 5 weight percent; reference to "minor concentrations"equates to from 5 to less than 25 weight percent and reference to "majorconcentrations" equates to at least 25 weight percent.

The particle sizes in the following examples are crystallite mean numberdiameters measured from approximately 100 particles in random 50,000×scanning electron microscopy images.

Example 1

A tungsten carbide-containing particulate precursor is prepared usingthe entrainment method described above, wherein the temperature of thereaction is held at 1550° C. (1823 K), the atmosphere is argon, thereaction time is about 2 to 4 seconds, the heating rate is about 10,000to 100,000,000 K/sec and the reactive particulate mixture consists of84.7 parts by weight of TO-3, (WO₃) and 15.3 parts by weight of Chevronacetylene black, as the source of carbon. 250 Grams of the resultingparticulate precursor is homogenized in a 1-liter urethane-lined ballmill with 5 millimeter WC--Co milling media for 30 minutes, sievedthrough a 30-mesh screen, milled again for an additional 30 minutes andsieved again through a 30-mesh screen. The homogenized particulateprecursor contains 1.29 weight percent of carbon and 4.12 weight percentof oxygen as measured by a combustion technique using apparatusmanufactured by Leco Corporation (St. Joseph, Mich.).

86.1 Parts by weight (pbw) of the particulate precursor, 3.6 ppw ofChevron acetylene black and 10.3 ppw of Co₃ O₄ (#22, 164-3 from AldrichChemicals, Milwaukee, Wis.) are milled together using the same millingprocedure described for the homogenization of the particulate precursorto form a finishing mixture. The finishing mixture is formulated toobtain a product that has a weight ratio of WC/Co of 92/8 whichcorresponds to a carbon concentration of a bout 5.64% by weight.

50 Grams of the finishing mixture are placed in a quartz boat and theboat is placed in a tube furnace for conducting a finishing reaction.The finishing reaction is conducted at 1100° C. (1373 K) for 120 minutesin a flowing atmosphere of 5 mole percent hydrogen in argon. The productfrom the finishing reaction contains WC and Co as shown by X-raydiffraction. The oxygen and carbon contents in the final WC--Co productare, respectively, 0.14 weight percent and 5.68 weight percent asmeasured by combustion analysis. 5.68 Weight percent is about the samecarbon concentration as stoichiometeric amount desired. Scanningelectron microscopic analysis of the product indicates that the averageparticle size is about 0.1 micrometer.

Example 2

Example 1 is repeated except the finishing mixture consists of 86.6parts by weight of the particulate precursor, 3.0 parts by weight ofcarbon, and 10.3 parts by weight of Co₃ O₄ and the finishing temperatureis 950° C. (1373 K). The formulation for the finishing mixture is basedon forming a product having a WC/Co weight ratio of 92/8. The finishedproduct has oxygen and carbon contents of 0.29 weight percent and 5.70weight percent, respectively, an average particle size of about 0.1micrometer, a major concentration of WC and a minor concentration of Co.

Example 3

Example 2 is duplicated except that the time for the finishing reactionis 12 minutes. The oxygen and carbon levels in the finished product are0.16 and 5.82 weight percent, respectively. X-ray diffraction analysisindicated that the final product had a major concentration of WC and aminor concentration of Co.

Example 4

In Example 4, the desired product is WC, WC--TiC--TaC solid solution andcobalt metal powder wherein the chemical composition of the powder as awhole has a molar ratio of 8(WC):1(WC--TiC--TaC):1(Co). In the absenceof any free carbon, the desired powder product has a carbonconcentration of about 7.2% by weight. The solid solution contains aboutequal weights of the carbides. That is to say, the molar formula for thesolid solution is approximately (WC-3.25(TiC)--TaC).

Tungsten trioxide (Scopino Yellow Oxide obtained from TACOW TradeConsultants, Ltd. Hockessin, Del.), tantalum pentoxide (Zhuzhou-GradeFTa205, also obtained from TACOW Trade Consultants, Ltd.), titaniumdioxide (Kronos K3020, obtained from Matteson-Ridolfi, Riverview, Mich.)and carbon black (Chevron Acetylene Black) are mixed by ball milling.The resultant reactant mixture contains 14.78 kg WO₃, 1.79 kg Ta₂ O₅,2.08 kg TiO₂ and 3.95 kg carbon black and is balled-milled for one hourin a 40-gallon ball mill that contains 400 lbs. of 0.5-inch (12.7 mm)diameter WC-6% Co milling media. After ball milling, said mixture ispassed through a coarse (8 mesh, 2.36 mm) screen to remove the millingmedia.

Twenty-two (22) kg of said mixture are loaded into a feed hopper of avertical graphite tube reaction furnace of the type disclosed in U.S.Pat. Nos. 5,110,565 and 5,380,688. The furnace tube is 3.35 meters longand has a 15.2 centimeter inside diameter. The feed hopper is connectedto a cooled reactant transport member of the furnace by a twin screwloss-in-weight feeder. The reactant transport member has an insidediameter of 1.3 cm and is maintained at a temperature of approximately283 K by water flowing through a cooling jacket. After the mixture isloaded into the feed hopper, the furnace tube is brought to atemperature of 2083 K in about 30 minutes as measured by opticalpyrometers viewing the outside wall of the reaction chamber of thefurnace tube. Argon gas flows into the reactant transport member at arate of 3 scfm (85.05 slm).

The reactant mixture is then fed from the feed hopper into the cooledreactant transport member at a rate of 10 kg per hour (22 lbs per hour)by the twin screw feeder. The flowing argon gas entrains the particulatemixture and delivers it to the reaction chamber as a dust cloud. Theparticulate mixture is immediately heated in the reaction chamber at arate of approximately 10,000 to 100,000,000 K per second causing acarbothermal reduction reaction to occur. The average residence time ofsaid mixture in the furnace is between 3 and 4 seconds.

After exiting the reaction chamber, the flowing argon and carbonmonoxide (which is generated during the carbothermal reduction reaction)gas mixtures carry the particulate precursor into a water-cooledstainless steel jacket that rapidly cools the precursor below 283 K.After exiting the reactor, the precursor is collected in a plastic bagthat is placed in a stainless steel drum. The precursor is homogenizedusing a ball mill as described in Example 1. The homogenized precursorcontains 2.39 weight percent oxygen and 6.78 weight percent carbon.

A finishing mixture containing 87.1 parts by weight of the precursor,3.0 parts by weight carbon, and 9.9 parts by weight Co₃ O₄ is made usingthe same milling procedure described in Example 1.

50 Grams of the finishing mixture are placed into a graphite tray andthe tray is placed into a graphite furnace for conducting the finishingreaction. The finishing reaction is conducted at 1350° C. (1423 K) for60 minutes in a flowing atmosphere of 5 mole percent hydrogen in argon.The product from the finishing reaction contains WC, a cubic solidsolution WC--TiC--TaC carbide and Co as determined by X-ray diffraction.The oxygen and carbon contents in the final product are 0.19 weightpercent and 6.60 weight percent, respectively. Scanning electronmicroscopy of the product indicates that the particle size is about 0.3micrometer. The product has a carbon concentration which is less thanstoichiometeric (about 7.2% by weight).

Example 5

In Examples 5-7, the desired product is WC, WC--TiC--TaC solid solutionand Ni metal powder wherein the chemical composition of the powder as awhole has a molar ratio of 8(WC):1(WC--TiC--TaC):1(Ni). In the absenceof any free carbon, the desired powder product has a carbonconcentration of about 7.2% by weight. The solid solution contains aboutequal weights of the carbides. That is to say, the molar formula for thesolid solution is approximately (WC-3.25(TiC)--TaC).

Example 4 is duplicated except that the finishing mixture contains 88.0parts by weight of the precursor, 2.5 parts by weight carbon, and 9.5parts by weight NiO and the finishing temperature is 1250° C. (1523 K).

The product contains WC, Ni and WC--TiC--TaC solid solution asdetermined by X-ray diffraction. The oxygen content is 0.95% by weightand the carbon content is 7.05% by weight. The carbon content is nearlythe desired stoichiometeric amount (about 7.2% by weight). The particlesize is about 0.1 to 0.2 micrometer.

Example 6

Example 5 is repeated except that the finishing mixture contains 87.5parts by weight of the precursor, 3.0 parts by weight carbon and 9.5parts by weight NiO.

The product contains WC, Ni and WC--TiC--TaC solid solution asdetermined by X-ray diffraction. The oxygen content is 0.70% by weightand the carbon content is 7.34% by weight. The carbon content is nearlythe desired stoichiometeric amount (about 7.2% by weight). The particlesize is about 0.1 to 0.2 micrometer.

Example 7

Example 6 is repeated except that the finishing reaction temperature is1350° C. (1623 K).

The product contains WC, Ni and WC--TiC--TaC solid solution asdetermined by X-ray diffraction. The oxygen content is 0.15% by weightand the carbon content is 6.90% by weight. The carbon content is belowthe desired stoichiometeric amount (about 7.2% by weight). The particlesize is about 0.2 micrometer. For all of the above examples, the carbonconcentration of the product is adjustable by the amount of carbon inthe finishing mixture, temperature of the reaction and, to a lesserextent, by the time of the reaction. The optimum reaction parameters areempirically determinable.

What is claimed is:
 1. A transition metal carbide-Group VIII metalpowder comprising a mixture having particles of a transition metalcarbide and Group VIII metal wherein:at least 50% by number of theparticles are discrete, the particles have at most an average aspectratio of about 1.5, substantially all of the particles have a size ofbetween 0.1 and 0.4 micrometer, the transition metal carbide is acarbide selected from the group consisting of tungsten, titanium,tantalum, molybdenum, zirconium, hafnium, vanadium, niobium, chromiumand solid solution thereof, the Group VIII metal is selected from thegroup consisting of iron, cobalt, nickel and solid solution thereof andthe transition metal carbide-Group VIII metal powder contains an amountof Group VIII metal of at least about 0.25% to at most about 50% byweight of the transition metal carbide-Group VIII metal powder.
 2. Thetransition metal carbide-Group VIII metal powder of claim 1 wherein saidpowder is a monotungsten carbide-cobalt powder.
 3. The transition metalcarbide-Group VIII metal powder of claim 2 wherein the monotungstencarbide-cobalt powder has a cobalt concentration of at least about 1% byweight of the powder.
 4. The transition metal carbide-Group VIII metalpowder of claim 1 wherein the size is at most about 0.2 micrometer indiameter.
 5. The transition metal carbide-Group VIII metal powder ofclaim 1 wherein said powder contains a free carbon amount that is atmost about 0.2% by weight of the powder.